Reduction of tube noises



Nov. 1, 1938. c. w. HANSELL. 2,135,034

REDUCTION OF TUBE NOISES I Filed Jan. 28, 1935 AUD/O OUTPUT INVENTORCLARENCE W. HA A/JELL BYWggWM/ ATTORN EY Patented Nov. 1, 1938 UNITEDSTATES REDUCTION OF TUBE Noises Clarence W. Hansell, Port Jefferson,N..Y., assignor to Radio Corporation of America, a corporation ofDelaware Application January 28, 1935, Serial No. 3,692

6 Claims.

This invention relates to a novel method of and circuit for operatingthermionic tubes including a method of and means for obtaining morequiet operation of thermionic tubes in relay or amplifier circuits thanhas been possible heretofore.

If in any tube there is a grid between the oath-- ode and anode which isoperated at positive potential then there will be a flow of current tothis grid in which should appear fluctuations corresponding to thefluctuations in current between the cathode and anode. If the circuitbetween this positively charged grid and the filament contains a highseries impedance adjacent the grid or such an impedance is in thefilament return or a circuit tuned to the frequency at'which the tube isbeing operated is placed in either of said positions, the fluctuationsin current flowing from the filament will be smoothed .out. by thechoking action and prevented, or reduced to a low value, and there willbe instead fluctuations in potential between the filament and said grid.This fluctuation in potential will tend to prevent variations in thecurrent flowing to and through the grid. That is, it will reduce theeffect ofalternating cathode heating current fluctuations in cathodeemission and initial electron velocities and shot effect.

' If the wanted variations, e. g., variations caused by or productive ofsignalling currents, in anode current also cause variations in electroncurrent leaving the cathode then the means described above acting on thegrid will tend to cause degeneration by opposing or reducingthis usefulflow as described above. This may reduce the useful output from the tubein the same ratio as the reduction in tube noise. However, I propose toso construct and use the tube that there is a storage of electrons orspace charge outside the grid which encloses the filament and the usefulanode current variation will flow from' this storage reservoir. In otherwords, the volume of space surrounding the space charge grid will act asa virtual cathode, spaced from the heated surface of the real cathode,insofar asthe useful functions of the tube are concerned. Now if thevariations in cathode emission are prevented or reduced as described inthe preceding paragraph there will be no degenerative effect on theuseful output which depends on the stored electrons rather than on rateof emission; This space will be fed with a constant flow of electronsfrom the filament equal to the average plate current while,

at the same time, it acts as a reservoir to accommodate the usefulvariations in plate current.

diminution of the charge during current peaks.

The novel features of my method and circuit have been set forth withparticularity in the claims appended hereto.

The nature of my invention and the mode of operation of thesame will bebest understood from the following. detailed description thereof whenread in connection with the drawing, throughout which like referencecharacters indicate like parts, and in which:

Figures 1 to '3 inclusive sho-W "various circuit arrangements forcarrying out the principles of my invention.

The invention is applicable to relays, amplifiers, modulators ordemodulators, and the potentials involved may be of any frequency.

Referring to the drawing, in Figure 1, I have shown a circuit by meansof which quiet and'effi- 'cient amplification of audio frequencies maybe obtained. 1

'In Figure 1, 2 is a thermionic tube having a control grid 4, aninner-grid 6, an anode 8, and a cathode 5. Qrdinarily these electrodeswill be constructed concentrically and in said structure the spacecharge grid 5 will surround the cathode. The control grid 4 is coupled,as shown, by way of a transformer H! to any source of low frequencyalternating current which :it is desired to amplify. The control grid 4is maintained at the desired negative potential by a battery Hconnected, as shown, with its negative terminal towards the control grid4 and its positive terminal to ground.

The inner-grid 6 may be maintained at positive potential with respect tothe cathode or may be connected to ground, as shown, while the cathode 5is maintained negative with respect to ground by means of the source Itconnected, as shown. The cathode 5 is energized by a source of energysuch as an accumulator or rectifier, or as shown, by output from analternating current transformer T, and the cathode energizing circuitincludes the audio frequency choke M which is of high impedanoetotheoscillations-from ID. If desired the cathode may be heated by aninsulated heater, inside a coated cathode, to separate the heating andcathode circuits.- In this case a choke may be included only in theconnection to the cathode. This choke has high impedance at thefrequency to be relayed, that is, at the frequency of the inputoscillations from l0 and prevents fluctuations in the electron currentflowing from the cathode. Consequently the audio frequency choke coilsM' prevent the noises which would normally result from the fluctuatingcathode emission. At the same time the direct current potentialdifference between the cathode 5 and the inner-grid 6 insures a constantflow of electron current to and through the grid 6 and, consequently, aspace charge or store of electrons is built up between the inner grid 8and outer grid 4. So long as the outer grid 4 remains at a constantnegative potential there will be a constant flow of current to theanode.

When the audio frequency potentials to be relayed are impressed ontheouter grid, the current to the anodeelectrode will vary also andthere will be a useful output from the tube which will appear in thetransformer I2. This output will be relatively free from noises producedby alternating current heating energy, or direct current on whichalternating current components are superposed, thermal agitation andvarying emission from the cathode. In this case the fluctuations inanode current are obtained primarily from fluctuations in the currentflowing to the space charge grid but not to any appreciable extent fromfluctuations in electron current from the cathode. At very highfrequencies the fluctuations will come primarily from the electron cloudor space charge outside the. space charge grid. Thus, at lowfrequencies, the space charge grid becomes a virtual cathode while atvery high frequencies the electron cloud or space charge near the spacecharge grid absorbs the current. fluctuations and becomes the virtualcathode.

Of course, the present invention maybe applied to a wide number ofcircuits and the circuit of Fig. 1 may be modified considerably withoutdeparting from the spirit of the present invention. For example, acircuit as shown in Fig. 2 including a neutralized amplifier tube, maybe used.

In Fig. 2 it is assumed that radio frequency potentials are to berelayed or amplified. In Fig. 2 the potentials to be amplified areapplied, as shown, by transformer 20 tothe control grid 4, and the radiofrequency oscillations may be derived from the output of transformer 22,connected with the anode of tube2. The control grid 4 is connected byway of the secondary winding of the transformer 20 'to ground, while thespace charge grid is connected directly to ground. The input and outputcircuits are, as shown, tuned to the radio frequency to be amplified.The cathode heating circuit includes radio frequency impedances 24connected, as shown, and tuned to the frequency of the oscillations tobe amplified by a tuning capacity 26.

In this modification the operation is the same as in the modificationshown in Fig. 1. The tuned impedances 24, prevent fluctuations in thecathode emission at the frequency of the oscillations to be amplified.The potential of the electrode 6 with respect to the cathode 5 builds upa space charge and stores up electrons in the space around the electrode6 which insure the desired useful stream of electrons to the anode toamplify or relay the potential variations. The capacity between theanode and grid electrodes may be balanced tothe desired degree by thecapacitor NC. V

The modification shown in Fig. 3 is similar in many respects to themodification shown in Fig. 2. Here, however, an additional screeninggrid electrode SG is interposed between the control grid 4 and the anodeto further insure quiet operation by preventing anode potentialvariations at the signal frequency to be amplified, from reaching thecontrol grid, space charge grid, or

cathode. The shielding grid SG is maintained at the desired positivepotential by connecting the same to the positive terminal of the sourceby way of a resistance 21, as shown. Radio frequency oscillations areby-passed around the resistance 27 and the source by means of by-passingcondenser C connected as shown.

Here, as in Fig.2, the inductances 24 in the cathode heating circuit aretuned by the capacitor 26 to the frequency of the oscillations to beamplified. Here, as in Fig. 2, it is assumed that radio frequencyoscillations are to be amplified, and the input and output circuits aretuned to the frequency of said oscillations.

Having thus described my invention and the operation thereof, what Iclaim is:

1. In a thermionic relay, an evacuated envelope enclosing a control gridelectrode, a cathode, an anode, and an auxiliary electrode locatedadjacent said cathode, a circuit for applying alternatingcurrentpotentials between the control grid and auxiliary electrode, aload circuit connected between the anode and cathode, a circuitconnecting said auxiliary electrode to said cathode, said circuitincluding means for maintaining said auxiliary electrode at a potentialrelative to said cathode such that a large flow of thermionic currentfrom said cathode to said auxiliary electrode is produced, and means forpreventing fluctuations in the current flowing from said cathodeincluding an inductive reactance in said auxiliary electrode-cathodecircuit of high impedance to current changes at the frequency of thealternating current to be relayed, said means also producing potentialvariations between said auxiliary electrode and cathode which tend toprevent fluctuations in current flow from said cathode to said auxiliarygrid.

2. In a thermionic relay, an evacuated envelope enclosing a control gridelectrode, a cathode, an anode, and an auxiliary electrode adjacent saidcathode, a tuned circuit for applying alternating current potentialsbetween the control grid and said auxiliary electrode, a tuned circuitconnected between the anode and cathode, said auxiliary electrode beingconnected with said cathode by means for maintaining said auxiliaryelectrode at a potential relative to said cathode at which there is alarge flow of electrons to said auxiliary electrode, and means forpreventing fluctuations in the flow of electrons from said cathodetosaid auxiliary electrode, including reactances in said auxiliaryelectrode-cathode circuit, said reactances being tuned to the frequencyof the alternating current to be relayed, said means also producingpotential variations between said auxiliary electrode and cathode whichtend to prevent fluctuations in current flow from said cathode to saidauxiliary grid.

.3. In a signalling system, an electron discharge tube having an anode,a control electrode, an auxiliary electrode, and a cathode adjacent saidauxiliary electrode, means for applying signalling potentials betweenthe control electrode and the auxiliary electrode, acathode heatingsource, means including a bias potential source in a circuit connectingsaid cathode heating source to said auxiliary electrode for maintainingsaid auxiliary electrode at a potential such that a large flow ofcurrent to said auxiliary electrode from said cathode takes place,whereby said auxiliary electrode forms a virtual cathode, and meansincluding an inductive impedance for pre venting the effect of, said'signalling potentials upon the emissiveness of said cathode, saidimpedance being connected at one end to said cathode and at the otherend to said circuit which interconnects the cathode heating source andthe auxiliary electrode biasing means.

4. In a signalling system, a tube having an output electrode, anelectron emission electrode, an auxiliary electrode adjacent saidelectron emission electrode and a control electrode,gmeans for applyingalternating current potentials to said control electrode, means forsupplying alternating current potentials from said output electrode aheating circuit for said electron emission electrode, means in saidheating circuit for converting fluctuations of the currenttherein intopotential variations, and means for maintaining said auxiliary electrodeat a potential relative to said emission element such that a largeconcentrationof electrons takes place on and around said auxiliaryelectrode whereby the same acts as a virtual cathode.

5. A system as recited in claim 4 wherein said means in said heatingcircuit for converting fluctuations in current therein into potentialvariations is an impedance the value of which is high with respect tothe alternating currents applied to said control grid. 7

6. A system as recited in claim 4 wherein said means in said heatingcurrent circuit for con pressed upon said grid circuit.

' CLARENCE w. HANSELL.

